Investigators involved in modern electronic and electromechanical industries increasingly have sought more refined and efficient devices and techniques in the generation of motion and the effectuation of its control. For example, the mass storage of data for popularly sized computers is carried out by recordation on magnetic disks which are rotatably driven under exacting specifications. The data handling performance of such memory handling systems relies to a considerable degree upon the quality and reliability of the rotational drive components associated with them.
Permanent magnet (P.M.) direct current (d.c.) motors generally have been elected by designers as the more appropriate device for refined motion generation or drive. Clasically, the P.M. d.c. motor is a three-phase device having a stator functioning to mount two or more permanent magnet poles which perform in conjunction with three or more rotor mounted field windings. These windings are positioned over the inward portions of pole structures typically formed of laminated steel sheets. The ends or tip portions of the rotor poles conventionally are flared or curved somewhat broadly to improve their magnetic interaction with the stator magnet. Field windings are intercoupled with either a delta or a Y circuit configuration and by exciting them in a particular sequence, an electromagnetic field, in effect, is caused to move from one pole tip to the next to achieve an interaction with the permanent magnet fields and thereby evolve rotatonal motion. This interaction occurs typically through an air gap which is generally in parallel relationship with or concentric to the axis of the rotor while flux transfer occurs across the gap radially. The interaction between the permanent magnet field and field of the excited windings is one wherein tangential force vectors are developed in consequence of an association of the exciting field with the field or flux of the permanent magnet. Clasically, the switching providing select excitation of the field windings is provided by a commutator rotating with the rotor and associated with brushes representing a make and break mechanical switching device functioning to move the field along the pole tips.
With the advent of more sophisticated electronic systems such as disk memory data storage assembly of computers, the classic P.M. d.c. motor has been found to be deficient in many aspects. For example, the make and break commutation of historic designs is electrically noisy and somewhat unreliable, conditions which are unacceptable for such applications. Such motors are relatively large, and this aspect contributes to undesired design requirements for bulk, the designer losing much of the desired flexibility for innovation in applications requiring motor drive. Further, the manufacturer of such motors must cope with the somewhat complex nature of typical rotor pole structures carrying field windings. For example, the production of the windings upon individual poles involves a procedure wherein wire is maneuvered beneath the flared tips of a fully assembled rotor structure.
To address the performance limitations of electrical noise caused by brush-type motors, brushless P.M. d.c. motors have been developed wherein field commutation otherwise carried out mechanically has been replaced with an electronic circuit. These motors generally provide a higher quality performance including much quieter electrical operation. A radial gap architecture is retained from earlier designs. As in earlier designs, the field windings are provided beneath flared pole tips but the flared pole tips are on the inside of the stator surface facing towards the central permanent magnet rotor poles and as such create a more difficult winding operation for the production of such motors. Typically, the permanent magnet components of such quieter electrical systems move as opposed to the field windings of the motor.
Where d.c. motors are configured having steel core poles and associated field windings performing in conjunction with rotor mounted permanent magnets, there occurs a somewhat inherent development of detent torque. At rest, or in a static state, the steel poles of a typical rotor will assume an orientation with respect to associated permanent magnets which develop flux paths of least potential energy which corresponds to positions of highest flux density and least reluctance. Thus, were one to hand rotate the rotor of an unenergized motor of such design, these positions of rest or detent positions can be felt or tactilly detected as well as the magnetic field induced retardation and acceleration developed in the vicinity of the detent positions. During an ensuing excitation state of the motor windings creating rotational drive, detent torque will be additively and subtractively superimposed upon the operational characteristics of the motor output to distort the energized torque curve, increase ripple torque, often reduce the minimum torque available for starting, and, possibly, develop instantaneous speed variations (ISV) which generally is uncorrectable, for example, by electronics. ISV characteristics also can be generated from mechanical unbalance phenomena in the rotor of the motor itself or the bearings thereof if they are a part of the rotated mass. Generally, detent torque contributions to ISV and other phenomena are observable in the operational characteristic or torque curve of motors, for example being manifested as a form of ripple torque. In the past, the dynamic output of the motors has been smoothed through resort to rotational mass such as flywheels and the like. However, for great numbers of modern applications, design constraints preclude such correction and motors exhibiting large ISV characteristics are found to be unacceptable. As a consequence, spindle motors for disk drives of computer systems, for example, have been configured as vector cross products or B cross I devices, sometimes known as voice coil motors, which do not employ steel pole structures in the air gap.
Petersen, in U.S. Pat. No. 4,745,345 entitled "D.C. Motor with Axially Disposed Working Flux Gap" issued May 17, 1988, describes a P.M. d.c. motor of a brushless variety employing a rotor-stator pole architecture wherein the working flux gap is disposed "axially" (to the motor axis) and wherein the transfer of flux is generally parallel to the axis of rotation of the motor. This "axial" architecture further employs the use of field windings which are simply structured being supported from stator pole core members which, in turn, are mounted upon a magnetically permeable base. The windings positioned over the stator pole core members advantageously may be developed upon simple bobbins insertable over the upstanding pole core members. Such axial type motors have exhibited excellent dynamic performance and efficiency and, ideally, may be designed to assume very small and desirably variable geometric configurations.
Detent torque characteristics which otherwise might occur with such motor designs are accommodated for by adjusting the geometric design of the permanent magnets within the rotor structure as well as, for example, by developing a skew orientation of the stator core poles. The latter skewing approach, however, necessarily is avoided where the noted design requirements for diametric miniaturization are encountered. Because of the static permanent magnet induced axial forces necessarily present with such motor structures, accommodation also may be necessary for such forces as well as for any time varying force term generated in consequence of commutation of the motors. Without such accommodation, for example, audible noise may be generated which for some applications will be found undesirable.
Petersen, in application for U.S. patent, Ser. No. 220,235 filed July 18, 1988, and assigned in common herewith describes a d.c. motor with an axial architecture wherein the noted permanent magnet induced axial forces are substantially eliminated through the employment of the axially polarized rotor magnets in a shear form of flux transfer relationship with the steel core components of stator pole positions. The dynamic tangentially directed vector force output (torque) of the resultant motor is highly regular or smooth lending such motor designs to numerous technological applications requiring both design flexibility, volumetric efficiency, low audible noise and a very smooth torque output.
A particularly desirable characteristic of the architecture wherein rotor magnets are associated with core components in a shear orientation resides in the provision of a localized balancing of the magnetic forces of attraction between the rotor and stator components. In classic motor designs, this balance is achieved in a diametric sense, a magnetic attraction at one side of the circumference of the rotor balancing that developed at the other side. With the axial shear motor approach, each magnetic interaction is locally balanced to provide refined rotor motion characteristics. Increasingly, the designers of products which necessarily incorporate D.C. p.m. motors are seeking smaller and smaller envelopes within which to incorporate given operational functions. As a consequence, motor structures are required which are of smaller size, yet which must achieve concomitantly higher volumetric efficiencies to maintain requisite torque outputs, i.e. torque per unit volume, high value acceleration characteristics and the like. Very often, the restricted space available to a motor function additionally is of irregular shape. Thus, the operational functions of the motors such as requisite magnetic flux return patterns, pole windings, associated stator core component designs, and the like are required to exhibit a packaging flexibility without unduly sacrificing motor performance requirements.